1. Field of the Invention
The present invention relates to a process for dehydrating aqueous ethanol from a conventional aqueous ethanol source to a desired concentration by means of high-pressure distillation and drying of the product alcohol, and then, if desired, utilizing the product ethanol to produce gasohol. More particularly, the present invention relates to a process for dehydrating aqueous ethanol from a conventional aqueous ethanol source, such as the aqueous ethanol product derived from a conventional fermentation process, utilizing a single high-pressure distillation to achieve a vapor phase, ethanol-water admixture containing the desired ethanol concentration (e.g., about 78 mole % or 90 weight % ethanol), and then drying the resultant vaporous admixture with a 3A type crystalline zeolite in the presence of CO.sub.2, utilizing the "pressure energy" derived from the distillation to allow the product ethanol to condense at ambient temperatures. If desired, the resultant dried ethanol product can be employed to make gasohol.
2. Description of the Prior Art
Dehydration of alcohols, particularly of ethanol, is a well-known art. Generally the methods used for dehydrating alcohols (and ethanol in particular) have either involved distillation of ternary azeotropes or non-distillation methods involving the use of various adsorbents. Many of the methods described in the patent literature have involved the addition of a third component to the binary water-ethanol azeotrope, which is soluble in water, but not ethanol, to product a ternary azeotrope, which is then subjected to distillation. The methods described in U.S. Pat. Nos. 2,140,694; 2,173,692; 2,358,193; 2,386,058; and 3,575,818; and in British Pat. No. 566,025; and in Polish Pat. No. 70,786--all fall within this category.
Additional specific examples for the dehydration of alcohols are set forth in a number of other representative patents. For example, U.S. Pat. Nos. 2,173,692; 2,358,193; and 3,575,818 describe the production of anhydrous ethanol by distilling a ternary azeotrope formed by adding diisopropyl ether, ethyl ether, pentane, and cyclohexane, respectively, to the binary azeotrope.
Non-distillation methods of dehydrating alcohols have centered upon the use of adsorbents, such as shown by U.S. Pat. No. 2,137,605; German Pat. No. 1,272,293; and Canadian Pat. No. 498,587, which collectively describe the use of either adsorbents or absorbents for drying ethanol including materials such as alumina, clinoptilolite, zeolite sodium-A, and bauxite, fuller's earth, and acid-activated bentonite.
While the foregoing patents are generally illustrative of the state of the art with respect to the dehydration of alcohols by either distillation or non-distillation means, it is also known in the art to combine these means under appropriate conditions. For example, U.S. Pat. No. 3,122,486 is concerned with an operation that combines a distillation operation with the use of a heatless fractionator for the purpose of treating aqueous azeotropic mixtures from a distillation unit, and this operation is said to be especially adaptable to production of high-purity anhydrous alcohols. The process of this invention consists of distillation of the crude alcohol to its azeotropic water mixture, followed by desiccation of the azeotrope by heatless drying over a water-selective adsorbent. Among the adsorbents taught by this patent are those that do not deteriorate when exposed to heat such as synthetic ion exchange resins. In general, such resins consist of polymers comprising a polystyrene that has been cross-linked with divinyl benzene and further treated, such as by sulfonation.
Also of some interest with respect to this combination of means is U.S. Pat. No. 3,132,079, which is directed to the removal of water from water-isopropanol mixtures to yield a pure isopropanol stream. In this patent, a distillation column is utilized to effect a substantial amount of separation of a crude alcohol feed; but, since the purity of the alcohol-rich product is limited by the alcohol-water azeotrope, the azeotropic mixture has to be removed as an overhead product and thereafter treated with an adsorbent. A variety of crystalline aluminosilicates (molecular sieves) are expressly mentioned as being equally useful for this purpose, including a large number of molecular sieves having pore openings in the range of about 3-10 Angstrom units. Alternatively, silica gel or alumina is also said to be utilizable as an adsorbent in such a system.
However, the latter two U.S. patents, although broadly suggestive of some aspects of the present process, nevertheless, are deficient with respect to the objectives, results, and means for obtaining same expressed in the present invention.
U.S. Pat. No. 3,122,486, for example, is not cognizant of the fact that: (1) high-pressure distillation should be used, and, if used, can be used to unexpected advantage as, e.g., in permitting the condensation of product ethanol to be obtained at ambient temperatures; and that (2) adsorption of the overhead from such high-pressure distillation can be conducted in the vapor phase, in the presence of CO.sub.2, thereby avoiding the disadvantages of co-adsorption such as is experienced in U.S. Pat. No. 3,122,486 and concurrently gaining the unexpected advantages mentioned above.
This patent, moreover, is dependent upon "pressure swing" in order to be operative, and also requires the use of a vacuum pump in order to obtain the pressure ratio required between adsorption and desorption. The present process is not subject to such deficiencies and limitations.
U.S. Pat. No. 3,132,079, in general, deals with thermal regeneration of the adsorbent it uses, and, in order to be operative, requires the use of a heater for its regeneneration process. In addition, this patent is unable to avoid the problem caused by co-adsorption of ethanol by its adsorbents, and also fails to appreciate the concept of water selectivity.
Not disclosed or suggested by the above patents, and unknown to the art, in general, however, are a number of important features which are characteristic of the present invention (and will be described in detail later) and which are also important to its successful practice; these features include, for example, (a) the use of carbon dioxide, preferably derived from the crude ethanol feed source and contained therein (e.g., in the form of a by-product), or N.sub.2, as a means with which to effect a number of concurrent functions such as the storage of the heat of adsorption, a means for temperature control and a desorption purge material with which to desorb the water impurity; (b) the use of a type 3A molecular sieve adsorbent (or natural crystalline zeolites having a pore size of 3 Angstroms) exclusively, since all other commercially available crystalline zeolites or molecular sieves, as well as carbon, alumina, and silica gel would co-adsorb ethanol and CO.sub.2 ; (c) the use of high pressure (i.e., 50 p.s.i.a. or more) distillation so as to permit condensation of the dried product ethanol at ordinary temperatures of, e.g., 80.degree. F.-100.degree. F.; (d) the absence of a need to utilize conventional swing adsorption, since a relatively low pressure ratio between the adsorption and desorption steps or stages can be utilized through the use of a circulating stream of CO.sub.2, etc.
The interaction of these and other features enables the present process of dehydrating ethanol to operate without need of any thermal energy beyond the energy used to the first stage distillation column, and only about 0.5% of the ethanol potential, in the form of electrical power, that is needed to totally dehydrate the ethanol from 90% to 100% by weight ethanol. The energy cost savings of this are so enormous as to render economically feasible and attractive the production of gasohol from the dried ethanol product produced in accordance with the practice of the process of this invention as an alternative to the production of gasoline.